1. Field of the Invention
The present invention relates to an optical element and a Mach-Zehnder optical waveguide element using the same for use, for example, in optical communications.
2. Description of the Related Art
Recently, the amount of information in optical communications has been increasing. In order to cope with the increase in the amount of information, measures such as the improvement of the signal speed and increase of the number of channels using wavelength multiplex communication have been taken in various type of optical communication networks such as backbone network, metro network, and access network. However, such measures require more complicating systems for optical communications and causes problems in size, price, and increase of power consumption.
In addition, similar measures against the increase of the amount of information are required in data centers of which the number has been increasing recently. In the conventional communications between computers in a data center, electric signals are transmitted mainly using metal cables. However, for further high speed communication and reduction of power consumption, optical communications using optical fibers have been applied recently. Moreover, for communications inside an instrument, the introduction of optical communications becomes a challenge at each level, for example, a board and a CPU.
As a technology for solving such problems in optical communication networks and achieving the application of optical communications to a new field, an optical device using high refractive index materials such as silicon has attracted attention.
The wavelength of light in a medium is inversely proportional to the refractive index of the medium. Therefore, with silicon (Si) having a high refractive index of approximately 3.5, the dimension of the core (width, height, and the like) of an optical waveguide will be smaller. In addition, when using as its cladding a medium such as silica (SiO2) of which the refractive index difference is large with respect to silicon, an optical waveguide with high confinement will be obtained. Such an optical waveguide will allow the bending radius to be smaller. Accordingly, it is possible to obtain a miniaturized optical device using an optical waveguide, and thus achieve further miniaturization with maintaining the functions, or accommodation of more functions (multi-functionalization) and higher density while maintaining the size.
Moreover, since high refractive index materials such as silicon are a semiconductor material, they are generally capable of being electrically controlled. With such semiconductor materials, it will be possible to realize characteristic-variable devices such as an optical modulator. Further, since optical devices in which a semiconductor is used for an optical waveguide core have a lot in common with conventional semiconductor devices such as CPU and memory in terms of technologies and equipment of manufacturing processes, it is expected to realize low-cost optical devices through mass production.
In addition, by integrating optical waveguides using semiconductor with conventional semiconductor devices using electrical signals on the same substrate, i.e., by replacing parts at which metal wirings are used with optical waveguides, it will be possible to achieve higher speed and lower consumption energy in instruments.
An optical modulator is one of major devices in optical communications for converting electrical signals into optical signals and has been studied by institutes as an element for achieving optical integrated devices as with the other devices.
The optical characteristics of silicon will be explained first. It is known that the refractive index of doped silicon in a communication wavelength region depends on the carrier density in the semiconductor. In accordance with R. A. Soref and B. R. Bennett, “Electrooptical effects in Silicon”, IEEE J. Quantum Electron. QE-23, 1987, p. 123-129, in P-type and N-type silicons, the change in refractive index (n) and the change in extinction coefficient (α) in silicon with respect to a light wavelength of 1.55 μm is expressed by the following Equation 1 and Equation 2, where the change in carrier density (numbers per 1 cm3) of electrons and holes are ΔNe and ΔNh, respectively.Δn=−[8.8×10−22×ΔNe+8.5×10−18×(ΔNh)0.8]  [Equation 1]Δα=8.5×10−18×ΔNe+6.0×10−18×ΔNh  [Equation 2]
Therefore, the higher the carrier density in the doped silicon becomes, the lower the refractive index becomes; and the lower the carrier density becomes, the higher the refractive index becomes. Also, the higher the carrier density becomes, the higher the extinction coefficient becomes; and the lower the carrier density becomes, the lower the extinction coefficient becomes. It is noted that the light wavelength in a medium is inversely proportional to the refractive index of the medium as described above. Therefore, when changing the refractive index without changing the length of the optical waveguide, the phase will be changed after transmission through the optical waveguide.
Accordingly, in a low extinction coefficient region, if doped silicon is used for an optical waveguide and the carrier density is changed in some way, it will be possible to change the phase after transmission through the optical waveguide due to the change in density change. Such an optical waveguide will functions as an optical phase modulator.
In view of the above, optical phase modulators have been proposed in which a PN junction or a PIN junction is formed in a rib waveguide (e.g., refer to International Publication No. 00/58776 (Patent Document 1), U.S. Pat. No. 7,085,443 specification (Patent Document 2), and U.S. Pat. No. 6,801,702 specification (Patent Document 3)).
In Patent Document 1, a PN junction having a vertical boundary is formed in the rib of the rib waveguide. Around the junction boundary of the PN junction, there is a region called a depletion layer where few carriers exist. When applying reverse bias voltage to the PN junction, the depletion layer will widen, resulting in decrease of carriers in the rib part. With taking advantage of the decrease, it will be possible to obtain an optical phase modulator in which the carrier density is controlled by voltage.
Patent Document 2 discloses an optical phase modulator having a PN junction and FIG. 2 thereof shows an example of its application to a waveguide in which strips made of polysilicon is loaded (Strip Loaded Waveguide).
Patent Document 3 discloses an optical phase modulator in which a PIN junction is formed.
For silicon or the like, because the loss changes at the same time as refractive index change when the carrier density is changed (voltage application), the insertion loss depends on the target phase of light.
In addition, in the configuration shown in FIG. 2 of Patent Document 2, the boundary of the material is in the center of the rib waveguide. A boundary of a material inside an optical waveguide may cause light diffusion in the advancing direction of light due to ununiformity (roughness) of the boundary face and increase of insertion loss in the optical waveguide.
The present invention was made in view of the above-described circumstances, and the object thereof is providing an optical element and a Mach-Zehnder optical waveguide element using the same which make it possible to reduce the amount of change in loss due to the change in carrier density and reduce the insertion loss.